1. Field of the Invention
The present invention relates generally to a milling machine and, more particularly, to a milling machine for milling a panel having complex contours.
2. Description of the Related Art
Many industries are and have been using sheet-like materials having complex contours. Industries use these materials for a wide variety of different purposes. For instance, the automobile industry primarily uses sheet-like, steel body panels. However, an increasing number of automobile body panels are being made from plastic, fiberglass, and composites, such as graphite and kevlar.
The steel sheets, from which steel body panels are formed, are typically produced using a roller press. The thickness of steel sheets formed in such a manner, may be controlled to within strict tolerances. However, plastic, fiberglass, and composite body panels are typically formed using molds. The molding process and any subsequent curing process often introduce irregularities in the thickness of these type body panels. Of course, in industries such as the automobile industry, body panels may vary in thickness without adversely affecting the quality or driveability of the automobile. In other industries, however, the thickness of body panels must be held to within strict tolerances. For instance, airplane manufacturers use composite panels to cover the fuselage and wings of an airplane. Composite panels are lighter and stiffer than steel panels and, thus, appear to be well-suited for use as skin panels on airplanes. However, variations in panel thickness may introduce undesirable stresses along the skin panel.
Due to material and processing variations during lay-up and curing, these large, integrally-stiffened skin panels rarely meet the stringent panel thickness requirements. These requirements are particularly important in fracture-critical zones, such as areas which attach the skin panels to an underlying substructure. The tolerance in these areas is approximately +/-0.010 inches for thickness, with an additional requirement that the waviness of the panel be held to 0.005 inches over any six-inch span.
As alluded to above, these composite panels are manufactured by stacking layers of fabric and tape, and then curing the layered materials in a high temperature/pressure autoclave. Due to variations in the thickness in the composite tape, pressures in the autoclave, and resin buildups or losses during curing, the panels often exhibit unacceptable levels of thickness variation. The thickness of the cured panels is measured to determine areas of undesirable thickness variation. Thereafter, thickness variations are eliminated by stacking more material in low spots and by manually grinding high spots. This process continues until the thickness of the panel is within the prescribed tolerance. This rework typically consists of manually inspecting the edge band with micrometers and pulse echo ultrasonic thickness gauges, and then chalking these readings onto the panel's inner mold line (IML) surface.
Typically, expensive hard tooling jigs and five-axis numerically-controlled machines perform the face milling operation. However, the inaccuracy of these machine tools and part-to-part variation make it nearly impossible to meet the required tolerance levels using these tools.
The present invention is directed to overcoming, or at least reducing the affects of, one or more of the problems set forth above.